Catalyzed gypsum wallboard and mutho j
of production thereof



My E. GERTON OF PRODUCTION THEREOF Original Filed DCC. G, 1962 March l2,1968 CATALY'ED GYPSUM WALLBORD AND METHO) INVENTOR. MARK E GERTON )ZW/.1M ATTORNEYS United States Patent Otiiice Re. 26,362 Reissued Mar. 12,1968 26 362 CATALYZED GYPSUh/I WALLBOARD AND METHOD OF PRODUCTIONTHEREOF Mark E. Gerton, Pleasanton, Calif., assigner to FibreboardCorporation, a corporation of Delaware Original No. 3,284,227, datedNov. 8, 1966, Ser. No. 242,806, Dec. 6, 1962. Application for reissueJune 22, 1967, Ser. No. 653,589

5 Claims. (Cl. 117-60) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

The present invention relates to compositions and improvements to drywall and plaster construction and more particularly to catalyzed gypsumlath and to a thin onecoat gypsum surface therefor.

More specifically, the present invention is directed to a process forinterior wall construction where gypsum wallboard or similar material isutilized as the wall base having a gypsum setting catalyst applied toits surface and a special thin coat of gypsum plaster is applied to thewall base to form a finished wall surface.

The use of thin gypsum plaster coatings on gypsum wallboard or lathprovides many advantages not previously found in conventionalconstruction. More specifically, the thin overall gypsum plaster coatprovides a harder surface that is much less porous than conventionalplaster construction. Thus the harder, denser, thin coat is more llameresistant and more sanitary than conventional plaster or dry wallsurfaces. In addition, much less shrinkage occurs, whereby splitting andcracking of the plaster surface is greatly reduced. The use of a singlethin coat also considerably reduces the total amount of material neededto finish the wall whereby great economies are realized. In addition,the thin gypsum plaster coat is applied in one operation, whereby laborcosts are also greatly reduced. Finally, the construction of theinvention provides a plaster surface coat which is more easily worked toobtain the finished surface and also provides an extremely tenaciousbond between the gypsum plaster coat and the wallboard backing.

While such thin wall coatings have been generally recognized as beingdesirable, achieving such a construction has been very ditlicult. Themajor problem in this respect is that thin plaster coatings placed uponconventional wallboard backing are subject to the defect known asdryout. Dry-out is the term applied to defective plaster wallconstruction wherein the water in the plaster coating is absorbed intothe wall backing before the plaster has set. Water is absolutelyessential to the setting of gypsum plasters: and if the water in theplaster is absorbed into the backing before the plaster can set, thenthe resultant coating becomes soft and powdery. In addition, the bond ofthe plaster coating to the backing is very poor. Thin plaster coatingsare particularly susceptible to dry-out. This is so, because the waterin the applied plaster formulation can be quickly evaporated and/oiabsorbed into the wallboard backing before the plaster has a chalice toset, whereby a dryout" occurs.

The present invention solves the dry-out problem and in addition, asnoted supra, produces a hard, dense adherent one-coat gypsum plastersurface on the wall. This is accomplished by specially preparing thewallboard backing during manufacture, More specifically, a catalyst isapplied to the surface of the wallboard backing. This catalystaccelerates the setting time of the gypsum plaster applied thereto;however the setting migrates from the wallboard backing surface throughthe plaster to the exterior. This rate of setting migration can beregulated by a proper selection of the accelerating agent. By suchmeans, the gypsum plaster sets initially at the plaster wallboardbacking interface, whereby a strong tenacious bond is achieved. Althoughthe gypsum plaster interface has already set, a period of time elapsesbefore the setting zone migrates to the plaster surface. A desirableconsequence of this fact is that the exterior plaster surface may beworked to achieve the desired appearance even though theplaster-wallboard interface has already set to form a strong bond.Therefore, any desired exterio.' surface finish can be achieved.

In addition to the catalyst treated wallboard backing, the presentinvention contemplates the use of a gypsum plaster composition whichsets to a hard, dense, nonshrinking coating most desired for suchconstruction and which may also be successfully applied to the backingin a very thin coat.

It is therefore an object of the invention to provide a wall having athin, one-coat plaster surface.

It is a further object of the invention to provide a plaster surfacedwall having a hard, dense surface.

It is yet another object of the invention to provide a thin coat plasterwall construction that is free from dryout problems.

It is still another object of the invention to provide a wallboardbacking, having a gypsum plaster catalyst applied to the exteriorsurface thereof.

It is yet another object of the invention to provide a gypsum plastercomposition which can be applied in a thin coat to a catalyzed wallboardbacking yet which provides a finished coating essentially free fromchecks and cracks.

Other advantages of the invention will become apparent upon reference tothe subsequent specification and claims as well as to the drawingappended hereto wherein:

The single figure is a graph illustrating the setting time of thin coatsof gypsum plaster as influenced by catalytic agents of varyingcomposition placed upon the wallboard backing.

The construction of the present invention comprises a conventionalwallboard or lath nailed or affixed by suitable means to the wallstudding. The wallboard or lath will generally be of the paper surfaced,gypsum-core type or of a compressed breboard material suitable for useas plaster backing. The wallboard or backing of the present inventiondiffers from that conventionally found in the art in that a suitablecatalytic agent has been impregnated into the outer surface thereof. Thecomposition of this catalytic agent is critical to the inventionconstruction. Thus, it has been found that a catalytic agent comprisingeither potassium sulphate or mixtures of potassium sulphate and aluminumsulphate are particularly suitable for the catalytic agent. Whilepotassium sulphate alone may be used, best results are obtained withmixtures of potassium and aluminum sulphate. Thus, on a dry basis, thecatalyst may comprise 10D-60 parts by weight of potassium sulphate and 040 parts by weight of aluminum sulphate,

Generally, for convenience, the catalyst mixture is applied to thewallboard during the manufacturing process. However, under certaincircumstances, the catalyst may be applied to the wallboard at any timesubsequent to the manufacture thereof and prior to the plasteringoperation. In any event, the catalyst is most conveniently applied tothe wallboard in the form of an aqueous solution. Advantageously. asmall amount of wetting agent such as sodium carbonate or otherswell-known in the art may be included in the solution of the catalyst ormay be applied separately to the surface of the wallboard in order tocondition it so that the solution of the catalyst will uniformly wet thewallboard. The aqueous solution of the catalyst is applied in aconcentration of at least 2 percent up to saturated (approximately 7-8percent).

The catalytic solution is applied to the wallboard at a rate of about0.3 to 1.0 lb. per thousand square feet of wallboard surface (dry weightof catalyst). As an illustration, wallboard according to the presentinvention has been produced using about 0.43 lb. of catalyst on adryweight" basis per 1,000 sq. ft. of wallboard surface. It should befurther noted that the catalyst solution should be a true solution andnot a slurry since a slurry would deposit too much catalyst on thesurface of the wallboard.

The catalyst solution is applied to the wallboard by any convenientmeans, such as by roller, brushing, spraying or by doctor blade. In anyevent, the catalyst composed of potassium sulphate alone or morepreferably of a mixture of potassium sulphate and aluminum sulphate isapplied to the wallboard surface at some time prior to the plasteringoperation.

The catalyst applied to the surface of the wallboard base solves thedry-out problem by accelerating the set of the gypsum plaster coating.Thus, the catalyst causes the gypsum plaster coating to set fairlyrapidly, with the set commencing adjacent the wallboard backing andmigrating slowly toward the surface of the gypsum plaster.

As a result, the gypsum plaster adheres firmly to the wallboard and yetthe outer surface can be worked for a reasonable length of time. In thisrespect, it should be noted that the aluminum sulphate in the catalystincreases the rate of setting of the gypsum plaster. Generally, however,it should not be used in combination with potassium sulphate in anyhigher a proportion than stated hereinbefore. Thus, too much aluminumsulphate would reduce the rate of water absorption into the wallboardbacking.

It is not desirable to completely prevent the water in the mixture frompenetrating into the wallboard, since the best finishes are obtained bytroweling the moist surface on the loss of water when its stitening isnot entirely due to the set alone. When troweling is accomplished as thewater is used up or penetrates into the board, the resultant smoothsurface will remain smooth as it sets from the inside. But if too muchwater remains in the gypsum plaster, it is diiicult to trowel smoothexactly at the time of the setting of the plaster. The loss of waterinto the wallboard backing is not harmful and it does not cause dry-outwhen the catalyst is applied to the board in accordance with thisinvention.

Tests were made to determine the effect of varying proportions ofcatalyst upon the setting time of gypsum plaster. The drawing presentedherewith illustrates the fact that higher proportions of aluminumsulphate added to potassium sulphate decrease the setting time of thegypsum plaster. The drawing also illustrates how the catalyst reducesthe setting time of the gypsum plaster with relation to uncatalyzedplasters. In fact, it should be noted that the gypsum plasterformulation utilized in the chart had a set of one hundred minutes whenuncatalyzed.

The final element of the wall construction of the present invention is athin coat of gypsum plaster applied to the catalyzed wallboard backingpreviously discussed. This gypsum plaster is a one-coat layer which canbe applied by troweling, spraying or other suitable means. In any event,the gypsum plaster coating is ideally kept very thin, preferably on theorder of 1/16" or less.

The thin gypsum plaster coating, which is applied to thecatalyst-treated lath has the following composition:

60-80 percent by weight gypsum -30 percent by weight inert filler, suchas sand, perlite,

shale, vermiculite, crushed marble or other similar materials 2-10percent of a clay such as re clay or china clay 0.5-2 percent of amixture of polyvinyl acetate and polyvinyl alcohol.

The proportions of materials set forth above are very important to theproperties of the gypsum plaster composition. Specitically, if less than60 percent of gypsum is used in the composition, the thin coat whenapplied to the wallboard backing will not have enough strength. On theother hand, if more than percent is used, there will not be enoughaggregate to provide a strong, crackfree finish.

Similar restrictions apply to the percentage of inert filler used in thegypsum plaster composition. The filler, regardless of whether it issand, perlite or any other similar material, should be less than 2-0mesh as measured on the U.S. Sieve series. In addition, if less than 15percent inert filler is used, the thin gypsum coat tends to developcrazing and cracking in the finish. On the other hand, if more than 30percent of the inert filler is used, the thin coat cannot be troweled toa smooth finish.

Clay in the gypsum plaster composition is necessary to provideplasticity for improved troweling properties when the surface isfinished. If less than 2 percent of clay is used it is ditiicult toobtain the desired plasticity in the thin coat. On the other hand, ifmore than 10 percent of the clay is used, the final surface is not ashard as it should be. In this respect, it should be noted that fire clayis preferred in the composition, since it develops the above-notedproperties in the gypsum composition to a greater extent than does chinaclay. However, china clay is still a perfectly acceptable material foruse in the composition.

The mixture of polyvinyl acetate and polyvinyl alcohol in the quantitiesnoted, supra, is extremely important to the gypsum plaster composition.The presence of polyvinyl acetate and polyvinyl alcohol in thecomposition imparts greatly improved toughness to the thin coat surfaceas well as partially retarding the nal drying thereof. It is believedthat final drying is retarded due to the formation of a film of thevinyl compositions on the surface of the thin coat gypsum plaster.Whether that is the specific mechanism by which the vinyl materialsretard the nal drying or not, it has been observed that they arespecically useful in this respect in the plaster composition.

If less than 0.5 percent of the polyvinyl mixture is used in the plastercomposition, the toughness properties imparted to the thin coat arelost. On the other hand, if more than 2 percent of the combinedpolyvinyl materials is included in the composition, the gypsum plastermixture is difficult to trowel and work when applied to the wallboardbacking. Bot"h polyvinyl acetate and polyvinyl alcohol should beemployed in the thin coat gypsum plaster mixture. The proportion ofpolyvinyl acetate to polyvinyl alcohol may vary from about l-2 to 2-1.Preferably, the composition should contain 4 parts by weight ofpolyvinyl acetate to 3 parts by weight of polyvinyl alcohol. Theseratios are the proportions of polyvinyl acetate to polyvinyl alcoholwithin the 0.5-2 percent combined limits previously set forth.

When desired, hydrated lime in the amount of from 0.25 to 2 percent byweight of the plaster mixture may be included. The lime balances theplasticity and reduces the tendency of nails and metal accessories torust. Lime in excess of 2 percent weakens the compressive strength ofthe plaster coat.

Under certain conditions, it is desirable to include a retarder in thegypsum plaster composition. Such retarders may be sodium citrate, citricacid, phosphates or any keratin product. The presence of the retarder inthe gypsum plaster mixture increases the working life of the plaster.For instance, when sodium citrate retarder is added in the amount ofabout 0.1 percent by weight in the gypsum plaster mixture, the workinglife is increased several times over that of the life of the plasterwithout the retarder therein. Thus, the plaster surface may be troweledsmooth over a longer period of time. Due to the presence of the catalyston the wallboard backing, while the surface is being troweled smooth,the plaster begins to set at the wallboard interface and the strong bondcreated holds the gypsum plaster on the wallboard backing securely whilethe outer surface is still workable. Where it is found desirable toincrease the working life of the applied thin plaster coat, retardermaterials as noted above may be added to the dry plastic composition inamounts not greater than 0.1 percent by weight.

In applying the gypsum plaster to the catalyzed wallboard backing. thedry composition is mixed with suficient Water to form a semisolidcomposition which can be applied in the usual manner. Such semisolidcomposition may be applied to the backing by hand or may be mixed With agreater amount of Water to have a consistency suitable for spray coatingonto the backing. As previously noted, the gypsum plaster is applied tothe `backing in thin coats preferably of 1734-, thickness or less. Suchthin coats provide a tough, hard. generally impervious surface over thewallboard backing in a single coat. The coating can be decorated bypainting or another suitable manner in twenty-four hours.

The thin coat gypsum plaster. catalyzed wallboard construction of thepresent invention is entirely suitable for use wherever conventionallath and plaster or other dry wall systems find application.

What is claimed is:

1. A method of wall construction comprising impregnating the surface ofa Wallboard backing with an aqueous solution of a gypsum settingcatalytic agent selected from the group consisting of potassium sulphateand mixtures of potassium sulphate and aluminum sillphatc, andplastering a single thin coat of gypsum plaster comprising about l5 to30 percent inert liller, about 2 to 10 percent plasticizer clay, about0.5 to 2% of a mixture of polyvinyl acetate and polyvinyl alcohol, andthe remainder consisting essentially of gypsum, upon the catalyzedsurface of said backing, and nal-nishing the thin plaster coat to form acontinuous plaster exterior surface.

2. The method of claim I wherein the concentration of the catalyticagent in the aqueous solution is between about 2 to 8 percent by weightand wherein the aqueous solution is applied to the backing at the rateof between about 0.3 to Il pound on a dry basis of catalyst per 1,000square feet of backing surface.

3 The method of claim l. wherein said thin plaster coat is applied onsaid Wallboard backing to a thickness of about 1,66 inch.

4. A wall construction comprising a wallboard backing, a gypsum settingcatalyst selected from the group consisting of `potassium sulphate andmixtures of potassium sulphate and aluminum sulphate impregnated in lllthe surface of said backing. and a continuous thin single coating ofgypsum plaster comprising about 15 to 30 percent inert filler, about 2to 10 percent plasticizer clay, about 0.5 to 2 percent of a mixture ofpolyvinyl acetate and polyvinyl alcohol, and the remainder consisting essenti-ally of gypsum, adhered to the exterior surface of said backing.

5. A wallboard backing for use with a thin one coat gypsum plastersurface comprising a core material and a fibrous surface material, and agypsum setting catalyst comprising a mixture of from about [1:2 to 2:1]3:2 to 39:1 parts by weight of potassium sulfate to aluminum sulfateapplied to said backing in the amount of about 0.3 to 1 pound ofcatalyst per one thousand square fect of backing surface.

References Cited The following references. cited by the Examiner. are ofrecord in the patented tile of this patent or the original patent.

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M. LUSIGNAN, Assistant Examiner.

